This invention relates to the comminution of lignocellulosic materials (referred to herein as “fibrous material” or “wood fibrous material”) and, particularly, to comminution using refiner plates having bars and grooves to separate fibers from lignocellulosic materials.
The invention is applicable to bar and groove designs for various types of refiner plates, including but not limited to disk refiners, counter-rotating disk refiners, twin and twin-flow refiners, cylindrical refiners, conical refiners and conical-disk refiners.
Refiner plates typically are arranged in a refiner to have facing surface separated by a gap. The plates rotate relative to each other. The fibrous material is introduced into the gap between the plates, typically, by flowing through a center inlet in one of the plates. The fibrous material flows in the gap between the plates and, in doing so, moves across the bars on the facing surfaces of the plates. As the fibrous material moves over the bars, the bars apply forces, such as compression pulses and impact forces, to the material. These forces tend to be greatest when the bars on the opposite plates cross over each other. The forces applied to the fibrous material act on the network of fibers in the material to separate individual fibers from the network and further develop these fibers. The separation of individual fibers and repeated compression of the fibrous mass results in the refining of the fibrous material.
Conventional refiner plates have refining bars separated by grooves arranged on a surface of the plate. The fibrous material, steam, water and other material flow through the grooves and over the bars as the material moves radially outward between the plates. Refining of the fibrous material tends not to occur in the groves. Refining occurs primarily as the fibrous material moves over the top ridges of the bars. The groves may include dams or other obstructions to prevent or restrict the flow of fibers and fluid through the grooves.
The bars typically include a sharp leading edge along a forward facing top edge of the bar. The conventional sharp leading edge angles of the bars are believed to promote shearing of the fibrous material passing over the bars. As bars on opposing plates pass each other, they impact and shear the fibrous material caught between the bars. The shear impacts of the fibrous material against the bar are a biproduct of the crossing of the bars. The shearing of fibrous material is undesirable.
Conventional wisdom views sharp leading edge angles as desirable to provide grooves with steep slopes such that the cross-sectional volume of the grooves provides sufficient flow capacity to move the fibrous material between the plates. A dull leading edge and its corresponding sloped leading face, i.e., leading sidewall, would result in conventional grooves having relatively narrow cross-sectional areas that may be insufficient to accommodate the flow of fibrous materials and the accompanying steam and water that should pass through the grooves. Examples of refiner plates with various types of leading edges on bars are shown in U.S. Pat. No. 5,039,022 entitled “Refiner Element Pattern Achieving Successive Compression Before Implact” and U.S. Pat. No. 4,678,127 entitled “Pumped Flow Attrition Disk Zone.”
The crossing of opposite bars creates compressive pressure pulses that impact the fibrous material between the bars. The compression pulses apply mechanical force to the fibrous material that promote the refining of the fibrous material. The compression pulses are believed to provide desirable refining action by producing high strength fibrous material.
There is a long felt need for refiner plates that minimize the impact forces and resulting shearing of fibrous material and maximize compression pulses to refine the material.